It must be remembered that the atmospheric pressure is liable to certain variations, and for this reason the dial of the aneroid will sometimes move when the instrument is stationary, indicating changes in atmospheric pressure. The atmospheric changes must be taken into consideration and corrected when the instrument is used for obtaining elevations.
The aneroid should be carried in the vest pocket at all times, to keep the temperature of the instrument constant. If the instrument is too large for the vest pocket it should be carried in the case with which all such instruments are provided. The work generally starts at a certain bench mark or at an assumed one.
Supposing that the elevations at the top or well mouth of three wells are wanted, the procedure is as follows: Starting at a bench mark whose elevation is assumed to be woo feet above sea-level, the barometer may either be adjusted so that the indicator will point to I000 feet or 'left as it is and the difference noted. It is best not to change the indicating hand, as the act of changing the position of the needle may unsettle the instru ment for the time being.
Standing at the bench mark with instrument in hand and holding same face up, the aneroid is tapped gently to allow the needle to be loosened to avoid "lagging"; care must be taken not to press the bottom of the instrument, as such pressure will be indicated by the needle. The needle reads 1120 feet at the bench mark which is 'coo. Record this reading in the note book as well as the time of the observation, which is assumed to be 9:30 A.M. From the bench mark go to the first well and repeatingthe method used at the bench mark, reading 1260 feet at 9:40. At the second well the readings are 'zoo feet at 9:so and at the third and last well 11oo feet at io:oo A.M. The next step is to go back to the original bench mark (or to any other bench mark that may be nearer), and the readings taken again, the instrument reads 1140 feet at 1o:30 at the bench mark. It will be noticed that the readings at the same bench with the same instrument at two different times shows a differ ence of 20 feet; which is due to the change in the atmospheric conditions in the interval. This difference is to be corrected for as follows: The error is +20 feet and the time interval one hour (6o minutes). The time interval divided by the error 60+ 20 = 3 ,
which is the number of minutes for each foot of change, i.e., for every three minutes the atmospheric change was one foot. Ten minutes after leaving the B.M. at the first well the error will be which is to be subtracted from the reading at pOint, which will be 1260-31 =12561. The second well reading taken at 9:5o, or 20 minutes after leaving the B.M.; the reading at that point is to be • diminished by 2o-i-3 =61; the corrected reading will be = 11931 feet. The third reading taken thirty minutes after leaving the B.M., the correc tion at that point will be 3o-f.3 = io, hence I Ioo— io= logo. The errors are to be subtracted (as in the above case) when the check reading at the bench mark is higher than the first reading. If the check reading should be lower than the first reading the calculations are the same, Vut the error is to be added in each case.
The next procedure is to take into consideration that the instrument was not set to read the exact elevation. The B.M. was moo feet and the instrument read 1120 feet. The instru ment reading being greater by 12o feet, this is to be subtracted from the corrected readings at each well, and the elevations so obtained will be the required elevation of each well. (Fig. 2o.) It must always be remembered that atmospheric changes will bring variations in the aneroid's movements, and therefore it must be constantly guarded against by checking at every opportunity at some bench mark. Every instrument has certain peculiarities with which the user must become acquainted. Some instruments have a certain amount of "lag," or in other words they do not catch up quickly with the change in elevation. This can be determined by several trials between two bench marks near each other and at different altitudes. A word of caution regarding the tapping of the instrument; a stiff needle will stand a great amount of tapping, but a sensitive one will respond very quickly, but unfortunately each tap will bring about different readings, so the best instrument is one that has a needle that is neither too sensitive nor too stiff.